gamma-delta T cells have unique features in comparison to alpha-beta T cells. It has now become clear that gamma-delta T cells recognize non-peptide and non-processed bacterial and environmental antigens, as well as stress-associated antigens expressed on epithelial cells and on certain tumor lines and primary carcinomas. However, understanding of gamma-delta T cell biology, especially cytokine responses, has lagged far behind that of alpha-beta T cells. Preliminary studies from our lab have shown that while polarization of native gamma-delta T cells in vitro can yield cells that produce IFN-gamma or IL-4 (like alpha-beta T cells), gamma-delta T cells default to a Th1 pathway, predominantly producing IFN-gamma, even in the presence of IL-4. Moreover, IL-12 signaling in gamma-delta T cells is constitutive and dissociated from IL-4-mediated regulation. Based on these data, the hypothesis is raised that the molecular mechanisms for gamma-delta T cell differentiation, especially the factors that modulate gamma-delta T cell polarization, are different from those in alpha-beta CD4+ T cells, and that gamma-delta T cells play important roles in early intracellular pathogen protection and in tumor immunity through their predominant production of IFN-gamma. To address these hypotheses, the following specific aims will be carried out: Aim l. To define the factors that regulate gamma-delta T cell IFN-gamma production in vitro. Aim 2. To define the factors that modulate gamma-delta T cell IL-4 production in vitro. Aim 3. To define in vivo the roles of gamma-delta T cells in regulating CD4+ T cell polarization. Aim 4. To define the role of IFN-gamma produced by gamma-delta T cells in pathogen protection and in tumor immunity. Study of the mechanisms of gamma-delta T cell differentiation in vitro and their functions in vivo is critical for understanding the function of gamma-delta T cells in immune responses.